ΔE 20.3, loose: different enough that you should pick on purpose. Darkness Grey is a little lighter, much more muted than Lupercal Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Darkness Grey is a little lighter, much more muted than Lupercal Green.
The same difference applies the other way round: Lupercal Green is a little darker, much more saturated.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Lupercal Green on the colour wheel See Darkness Grey on the colour wheel
All matches: Lupercal Green equivalents · Darkness Grey equivalents